In a spread spectrum communication system, the transmitted signal is spread over a frequency band that is significantly wider than the minimum bandwidth required to transmit the signal. By spreading transmission bandwidth across a broader bandwidth than minimally required, power levels at any given frequency within the bandwidth are significantly reduced. In one type of spread spectrum communication system, known as a direct sequence code division multiple access (CDMA) modulation system, a radio frequency (RF) carrier is modulated by a known digital code sequence referred to as a spreading code. The spreading code has a bit rate, or chipping rate, that is much higher than a clock rate of the underlying information signal. The RF carrier may be binary or quadrature modulated by one or more data streams. The data streams have one phase when the spreading code represents a data “one” or “high” and a predetermined phase shift (e.g., 180° for binary modulation and 90° for quadrature modulation) when the spreading code represents a data “zero” or “low”. These types of modulation techniques are commonly referred to as binary shift key modulation (BPSK) and quadrature shift key modulation (QPSK), respectively.
To retrieve the data encoded in a transmitted signal upon being received as a received signal, CDMA systems despread received signal samples with the same spreading code as was used to encode the data. CDMA systems employ a filter for correlating the spreading code with received signal samples to determine the received signal samples in a multi-chip data sequence.
A low power consuming analog filter for CDMA systems is disclosed in Low-Power Consuming Analog-Type Matched Filter for DS-CDMA Mobile Radio, by M. Sawahashi, F. Adachi, G. Shou, and C. Zhou, published in IEICE Transactions Fundamentals, Vol. E79-A, No. 12, December 1996, pp2071–2077.
In one known matched filter, multi-bit received signal samples are clocked through a delay line comprised of a plurality of delay stages, such as a shift register. The delays in the shift register are of a multiple bit width sufficient to accommodate the received signal samples. Each of the delay stages provides a delay of less than one-half of the period of the spreading code clock, or chipping rate, to satisfy the Nyquist sampling theorem. The signal samples propagate down the delay line through a series of successive shifts at a rate corresponding to the chipping rate. Taps at each delay stage provide the delayed signal samples for multiplication by respective tap weights associated with the respective delay stages to produce product terms, which when summed, provide the filter output. The tap weights represent the spreading code used to encode information on the transmission signal prior to being transmitted. The filter output is a correlation of the received signal samples with the spreading code represented by the tap weights.
Each time a new received signal sample is clocked into the delay line, each stage of the delay line is clocked to shift the previously received signal samples along the delay line by one delay stage. A received signal sample that has shifted completely through the delay line is shifted out of the delay line, as is known in the art. A shortcoming of this straight-forward implementation is the amount of power consumed by the delay line due to each stage of the delay line being clocked each time a new received signal sample is introduced into the delay line.
U.S. Pat. No. 6,075,807 teaches a digital matched filter for CDMA systems that include a digital delay line having a plurality of successive delay stages adapted to receive a digital signal and propagate the digital signal therethrough at a fixed rate. A correlator is coupled to the delay line to correlate the digital signal to a predefined spreading code to provide a correlation signal representing a degree of correlation of the digital signal to the spreading code. A window logic unit is coupled to the correlator to enable operation of the correlator only during successive discrete time periods of the correlation signal corresponding to a high degree of correlation of the digital signal to the spreading code.